As the drive power sources for portable electronic equipment such as mobile telephones (including smartphones), portable computers, PDAs, and portable music players, much use is made of alkaline secondary batteries and nonaqueous electrolyte secondary batteries, typified by nickel-hydrogen batteries and lithium ion batteries, respectively. Furthermore, alkaline secondary batteries and nonaqueous electrolyte secondary batteries are also much used as drive power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs), and in stationary storage battery systems in applications for curbing output variation of photovoltaic power generation and wind power generation, etc., in grid power peak load shifting applications for storing power at night and using it in the daytime, and in other applications. Particularly in EV, HEV and PHEV applications or stationary storage battery systems, high capacity and high output characteristics are required. Individual batteries accordingly get larger and are used connected in series or in parallel. Prismatic secondary batteries are widely used in such cases, because of their space efficiency.
Materials extremely rich in reactivity are used for the batteries in such applications, and particularly for nonaqueous electrolyte secondary batteries. Consequently, such batteries are required to have much higher safety than the secondary batteries used for small-sized portable equipment. Therefore, prismatic secondary batteries that are used for applications of the foregoing kinds are provided not only with a gas escape valve for releasing the battery outer casing internal pressure when it increases, but also with a current interruption mechanism for breaking the electrical connection between the external terminals and the electrode assembly inside the outer casing—as set forth, for example, in JP-A-2008-66254, JP-A-2008-66255 and JP-A-2010-212034.
For example, JP-A-2008-66254 discloses the invention of a prismatic secondary battery 50 that, as shown in FIG. 8A, includes an external terminal 53 having a through-hole 52 putting a current interruption mechanism 51 in communication with the space exterior to the prismatic secondary battery 50, and is so configured that the current interruption mechanism 51 is reliably actuated when the pressure inside the outer casing 54 increases. Furthermore, JP-A-2008-66255 discloses the invention of a prismatic secondary battery 60 that, as shown in FIG. 8B, includes an external terminal 63 having a through-hole 62 putting a current interruption mechanism 61 in communication with the space exterior to the prismatic secondary battery 60, and is so configured that the current interruption mechanism 61 is actuated when the pressure inside the outer casing 64 increases, and configured that the through-hole 62 is sealed by a membrane plug 65 of resin, in order to prevent moisture or oxygen from entering the current interruption mechanism 61 through the through-hole 62 and causing deterioration of the current interruption mechanism 61.
In the prismatic secondary batteries disclosed in JP-A-2008-66254 and JP-A-2008-66255, the through-hole is provided so that the battery exterior is in communication with the space in the current interruption mechanism that corresponds to the outside of the battery, and hence that the current interruption mechanism will be readily actuated when the pressure inside the outer casing increases. However, even if the pressure inside the outer casing increases due to some cause, the pressure of the gas that is produced in the battery interior will be extremely high during the abnormality, and there will be no simultaneous similar increase in the pressure inside the sealed space in the current interruption mechanism that corresponds to the outside of the battery. This means that there will be no substantial difference in the actuation of the current interruption mechanism, whether the space in the current interruption mechanism that corresponds to the outside of the battery is sealed or open.
JP-A-2010-212034 therefore discloses a prismatic secondary battery 70 that, as shown in FIG. 9, has a sealing body 71 that seals the mouth of the outer casing (omitted from the drawing), and a connection terminal 72 that is installed to the sealing body 71, with the object of rendering it difficult for electrolyte or cleaning fluid to enter the inside of the current interruption mechanism during manufacture. In this prismatic secondary battery 70, a current interruption mechanism 74 that interrupts the current in response to an increase in the pressure inside the outer casing is provided between the connection terminal 72 and a collector 73 that electrically connects the connection terminal 72 to the electrode assembly (omitted from the drawing); the connection terminal 72 has a through-hole 75 formed in its interior, the through-hole 75 which communicates with the space in the current interruption mechanism 74 that corresponds to the outside of the battery; and the through-hole 75 is sealed by a terminal plug 76 formed of an elastic member, so that a sealed space is formed between the through-hole 75 and the current interruption mechanism 74.
This current interruption mechanism 74 includes an inversion plate 77 that performs the function of a valve body, and the thin portion 73a of the collector 73. An annular groove 73b is formed in the thin portion 73a of the collector 73, and the inversion plate 77 is welded to the central part of the thin portion 73a. Moreover, the edge portion 77a around the periphery of the inversion plate 77 is welded to the inner circumferences of a flange portion 78a formed at the bottom end of the tubular portion of a tab member 78. The connection terminal 72 is electrically insulated from the sealing body 71 with an upper first insulating member 79 and a lower first insulating member 80 interposed therebetween, and is electrically connected to the top end of the tubular portion of the tab member 78. A second insulating member 81 of resin is disposed between the collector 73 and the inversion plate 77 at the periphery of the current interruption mechanism 74, and this second insulating member 81 is fixed to and integrated with the lower first insulating member 80 by latching-fixing portions 81a. As a result, when the pressure inside the outer casing increases, the inversion plate 77 is deformed toward the sealing body 71, and then the thin portion 73a of the collector 73 is cut through at the groove 73b. The electrical connection between the collector 73 and the inversion plate 77 is thus broken. This has the effect of stopping any further charging or discharging of the battery.
The prismatic secondary battery disclosed in JP-A-2010-212034 has high safety because it includes a current interruption mechanism. Moreover, during manufacture, the nonaqueous electrolyte or cleaning fluid, etc., will be unlikely to enter the current interruption mechanism. Thus, this invention offers the excellent advantages of a prismatic nonaqueous electrolyte secondary battery that includes high-reliability connection terminals.
However, in the event that the battery is subjected to shock due to vibration, falling, etc., the electrode assembly may shift toward the sealing body. In this case, the collector, which is connected to the electrode assembly, will also shift toward the sealing body. In the prismatic secondary battery 70 disclosed in JP-A-2010-212034, the shifting of the collector 73 toward the sealing body 71 will result in the collector 73 entering the interior of the tubular portion of the tab member 78, so that pressure will act on the welds between the tab member 78 and the inversion plate 77. This may cause fractures, cracks or the like in the welds between the edge portion 77a around the periphery of the inversion plate 77 and the inner circumference of the flange portion 78a formed at the bottom end of the tubular portion of the tab member 78. If fractures, cracks or the like are present in the welds between the inversion plate 77 and the flange portion 78a, the gas that is produced in the vicinity of the electrode assembly could enter the space inside the tubular portion of the tab member 78 through the fractures or cracks. The inversion plate 77 could fail to be deformed toward the sealing body 71 even if the pressure inside the outer casing increases, and the current interruption mechanism could cease to operate normally.
The present inventors arrived at the present invention upon discovering, as a result of many and various experiments to verify the causes of possible occurrence of fracturing in the welds of prismatic secondary batteries, that whether such fracturing takes place depends on the sizes of the inversion plate 77 and the collector 73 disposed opposing it, and on how they are disposed.